Engine stalls on 1985 MGM when a/c is running 302 CFI

[sdc71]

I took my 1985 Grand Marquis out for the first time year. I drove into the city about 40 miles away with a/c on and drove around town for an hour, no problems. I then drove to a small town about 15 miles, at every stop sign the car kill. It will start but when I put in Drive the engine will kill again and again. I would start it again and give a little gas then shift it into gear, before it would kill to get going again.

I had my mechanic adjusted the idle, but that did not work.

I appreciate all the help on this. 1985 MGM 302 CFI


Pictures from today, car is a little dirty 3 miles of gravel roads to my house.


.

[gadget73]

Does the throttle kicker work? If not, it will not bump the idle to compensate for AC load. The kicker is the thing right in front of the throttle lever. It should extend when you turn on the AC and push the throttle open a bit. If its not working, make sure the vacuum line to it is good, and check the other vac lines at the solenoids on the valve cover.

[sdc71]

Does the throttle kicker work? If not, it will not bump the idle to compensate for AC load. The kicker is the thing right in front of the throttle lever. It should extend when you turn on the AC and push the throttle open a bit. If its not working, make sure the vacuum line to it is good, and check the other vac lines at the solenoids on the valve cover.

Thank so much for the info. I will pass it on to my mechanic. I am also got an accident 4 or 5 years ago when it stalled at a very busy intersection. That is why I don't drive it that much, I only put on 200 miles in 2017. Any and all other suggests will be greatly appreciated.

[gadget73]

If this has never had a timing chain done, you're due. These have a plastic cam gear and it fails from age as much as mileage. When the chain gets slack it also develops some bad running habits. Vacuum leaks cause problems too.

[DerekTheGreat]

What gadget said about the dash pot which controls the throttle. x2 on making sure the vacuum lines are routed correctly. I had an '85 and my vacuum lines weren't right and the dash pot wasn't even in use. The idle was adjusted to something like 1,200 rpm to compensate for a sticky/dirty EGR valve. Cleaned that up, put a new TPS on it and was able to adjust the idle back down to what the sticker said it should be in gear. The dashpot in question lives on the driver's side of the throttle body. I've heard the sensitivity of it can be adjusted too. It is possible the diaphragm in yours is toast. When you turn on the A/C, you should see the plunger thing move out to increase the idle. Primitive way of doing it (The V6 CFI cars did have a true IAC) but that's how it was.

[kishy]

I have pulled every CFI VOTM (Vacuum Operated Throttle Modulator aka Dashpot) from every CFI junkyard car I've found in the last few years, and every single one leaks vacuum, plus the two on cars I own. I'm starting to wonder if there is a calibrated leak in them and they aren't supposed to actually hold it. It is almost certainly possible to rebuild it with the diaphragm from a new one of the same diameter but with a different bracket (and I believe certain carbs use exactly that).

For reference (not that you'll find one!) the part numbers for this little bastard are:

Ford E3VY9S520A
Motorcraft CD-69
CarQuest 733109
Standard D5131
Wells aka Duralast DP133
Wells again 1D1017

Agree, that at this time, the function of the VOTM needs to be verified and the reliability of the vacuum supply to it. It could be a double-pronged effect where the VOTM no worky due to bad vacuum supply and EEC-IV isn't liking the load of A/C at idle when a vacuum leak exists.

Also agree that a plastic timing gear is not so hot and if the last complete timing set replacement is not known or believed never to have been done, it should be planned.